Organic Thin Film Transistors

ABSTRACT

An organic thin film transistor comprising: a substrate; a source electrode and a drain electrode disposed over the substrate with a channel region therebetween; a layer of organic semiconductor disposed in the channel region; a gate electrode; and a gate dielectric disposed between the layer of organic semiconductor and the gate electrode, wherein the gate dielectric comprises a cross-linked polymer and a fluorine containing polymer.

FIELD OF INVENTION

The present invention relates to organic thin film transistors.

BACKGROUND OF THE INVENTION

Transistors can be divided into two main types: bipolar junctiontransistors and field-effect transistors. Both types share a commonstructure comprising three electrodes with a semi-conductive materialdisposed therebetween in a channel region. The three electrodes of abipolar junction transistor are known as the emitter, collector andbase, whereas in a field-effect transistor the three electrodes areknown as the source, drain and gate. Bipolar junction transistors may bedescribed as current-operated devices as the current between the emitterand collector is controlled by the current flowing between the base andemitter. In contrast, field-effect transistors may be described asvoltage-operated devices as the current flowing between source and drainis controlled by the voltage between the gate and the source.

Transistors can also be classified as p-type and n-type according towhether they comprise semi-conductive material which conducts positivecharge carriers (holes) or negative charge carriers (electrons)respectively. The semi-conductive material may be selected according toits ability to accept, conduct, and donate charge. The ability of thesemi-conductive material to accept, conduct, and donate holes orelectrons can be enhanced by doping the material. The material used forthe source and drain electrodes can also be selected according to itsability to accept and inject holes or electrodes. For example, a p-typetransistor device can be formed by selecting a semi-conductive materialwhich is efficient at accepting, conducting, and donating holes, andselecting a material for the source and drain electrodes which isefficient at injecting and accepting holes from the semi-conductivematerial. Good energy-level matching of the Fermi-level in theelectrodes with the HOMO level of the semi-conductive material canenhance hole injection and acceptance. In contrast, an n-type transistordevice can be formed by selecting a semi-conductive material which isefficient at accepting, conducting, and donating electrons, andselecting a material for the source and drain electrodes which isefficient at injecting electrons into, and accepting electrons from, thesemi-conductive material. Good energy-level matching of the Fermi-levelin the electrodes with the LUMO level of the semi-conductive materialcan enhance electron injection and acceptance.

Transistors can be formed by depositing the components in thin films toform thin film transistors. When an organic material is used as thesemi-conductive material in such a device, it is known as an organicthin film transistor (OTFT). OTFTs may be manufactured by low cost, lowtemperature methods such as solution processing. Moreover, OTFTs arecompatible with flexible plastic substrates, offering the prospect oflarge-scale manufacture of OTFTs on flexible substrates in aroll-to-roll process.

Various arrangements for organic thin film transistors are known. Onesuch device is an insulated gate field-effect transistor which comprisessource and drain electrodes with a semi-conductive material disposedtherebetween in a channel region, a gate electrode disposed adjacent thesemi-conductive material and a layer of insulating material disposedbetween the gate electrode and the semi-conductive material in thechannel region.

An example of such an organic thin film transistor is shown in FIG. 1.The illustrated structure may be deposited on a substrate 1 andcomprises source and drain electrodes 2, 4 which are spaced apart with achannel region located therebetween. An organic semiconductor (OSC) 8 isdeposited in the channel region and may extend over at least a portionof the source and drain electrodes 2, 4. An insulating layer 10 ofdielectric material is deposited over the organic semi-conductor 8 andmay extend over at least a portion of the source and drain electrodes 2,4. Finally, a gate electrode 12 is deposited over the insulating layer10. The gate electrode 12 is located over the channel region and mayextend over at least a portion of the source and drain electrodes 2, 4.

The structure described above is known as a top-gate organic thin filmtransistor as the gate is located on a top side of the device. Ifsource/drain contacts are located under the OSC layer the device may bemore completely described as a top gate, bottom contact device. Topgate, top contact devices are also possible, with the source/draincontacts located over the OSC.

Alternatively, it is also known to provide the gate on a bottom side ofthe device to form a so-called bottom-gate organic thin film transistor.

An example of such a bottom-gate organic thin film transistor is shownin FIG. 2. In order to show more clearly the relationship between thestructures illustrated in FIGS. 1 and 2, like reference numerals havebeen used for corresponding parts. The bottom-gate structure illustratedin FIG. 2 comprises a gate electrode 12 deposited on a substrate 1 withan insulating layer 10 of dielectric material deposited thereover.Source and drain electrodes 2, 4 are deposited over the insulating layer10 of dielectric material. The source and drain electrodes 2, 4 arespaced apart with a channel region located therebetween over the gateelectrode. An organic semiconductor (OSC) 8 is deposited in the channelregion and may extend over at least a portion of the source and drainelectrodes 2, 4.

If the source/drain contacts are located under the OSC layer, the devicemay be more completely described as a bottom gate, bottom contactdevice. Bottom gate, top contact devices are also possible, with thesource/drain contacts located over the OSC.

The conductivity of the channel can be altered by the application of avoltage at the gate. In this way the transistor can be switched on andoff using an applied gate voltage.

The dielectric layer comprises a dielectric material selected frominsulating materials having a high resistivity. The dielectric constant,k, of the dielectric is typically around 2-3 although materials with ahigh value of k are in principle desirable because the capacitance thatis achievable for an OTFT is directly proportional to k, and the draincurrent I_(D) is directly proportional to the capacitance. However,research has also shown that in many cases the properties of the OTFT isimproved with lower k dielectric materials, as is the case with thefluorinated materials discussed below.

The drain current that is achievable for an organic thin film transistoris inversely proportional to the thickness of the dielectric in theactive region of the device (channel between source and drainelectrodes). Thus, in order to achieve high drain currents with lowoperational voltages, organic thin film transistors must have thindielectric layers in the channel region.

It is apparent from the above that the dielectric in an organic thinfilm transistor, and the interface it forms with the OSC, is animportant factor in determining the operating characteristics of theorganic thin film transistor. As such, various materials and structuresfor the dielectric have been proposed in the prior art.

U.S. Pat. No. 6,265,243, which was published in 2001, discloses an OTFTin which the dielectric is surface-treated with a fluorinated organicmaterial such as a fluorinated silane. Suitable materials for thedielectric are given as silicon dioxide, polyimide, and polyvinylphenol(PVP). It is also disclosed that as an alternative to treating thedielectric surface with fluorinated organic material such as fluorinatedsilane, the dielectric material may be replaced with a dielectricpolymeric material rich in fluoroalkyl chains. No examples appear to begiven for this alternative dielectric polymeric material.

Applied Physics Letters, vol. 85, no. 12, p 2283 (2004) describes abilayer dielectric. This document describes using a bilayer comprisingPVP and polyvinyl acetate as the two layers.

U.S. Pat. No. 7,279,777, which was published in 2007 and refers to thepreviously mentioned document U.S. Pat. No. 6,265,243 in its backgroundsection, discloses substantially non-fluorinated cyano-functionalpolymers for the dielectric layer which preferably include acrosslinkable group. Various non-fluorinated styrene-containing surfacemodifying polymers are also disclosed for use with the non-fluorinatedcyano-functional polymers. It is described that much higher mobilitiesare achieved than for the fluorinated dielectrics disclosed in U.S. Pat.No. 6,265,243.

It is one aim of embodiments of the present invention to provide asolution to one or more of the problems discussed above.

SUMMARY OF INVENTION

The present applicant has determined that there are a number ofphysical, chemical, and electrical characteristics which it would beadvantageous for an OTFT gate dielectric to possess.

In terms of physical characteristics, it would be advantageous for thedielectric to be thin in order to achieve high drain currents with lowoperational voltages while also providing a robust, continuous layerwith no pin-hole defects, have low permeability to air, moisture, andother chemical contaminants, and provide a good, defect free interfacewith the organic semiconductor of the OTFT. It would also beadvantageous for the dielectric to provide good adhesion to the gate toprevent delamination while also providing a low contact angle surfacefor the organic semiconductor in order to prevent pinning of organicsemiconductor molecules to the surface such that they are more free tocrystallize increasing mobility. Furthermore, it would be advantageousfor the dielectric to be readily deposited to form a film having theaforementioned characteristics without damaging underlying layers.

In terms of chemical characteristics, it would be advantageous for thedielectric to be chemically inert with respect to air, moisture, andother chemical contaminants, and also provide no chemical contaminationof the organic semiconductor of the OTFT.

In terms of electrical characteristics, it would be advantageous for thedielectric to have a high resistivity in order to prevent currentleakage and a high dielectric constant k to maintain high gatecapacitance and achieve high drain currents. At the same time, it wouldbe advantageous for the dielectric to have a low k surface at theinterface with the organic semiconductor as it has been found that low kdielectrics improve the morphology of the organic semiconductor and havea reduced number of charge traps compared to high k dielectric surfaces.

Having identified the numerous advantageous characteristics which itwould be desirable for a dielectric to possess, the present applicanthas assessed the prior art arrangements and come to the conclusions thatnone of the prior art arrangements fully address all the desiredrequirements. The best dielectrics disclosed in the prior art discussedin the background section would appear to be those disclosed in U.S.Pat. No. 7,279,777. As previously indicated in the background section,these comprise substantially non-fluorinated cyano-functional polymersfor the dielectric layer which preferably include a crosslinkable group.Various non-fluorinated styrene-containing surface modifying polymersare also disclosed for use with the non-fluorinated cyano-functionalpolymers. It is described that much higher mobilities are achieved thanfor the fluorinated dielectrics disclosed in U.S. Pat. No. 6,265,243.

The present applicant has realized that the preferred crosslinkablepolymers disclosed in U.S. Pat. No. 7,279,777 would meet many of thedesired criteria they have identified.

Cross linkable polymers generally have a higher degree of physical,chemical and thermal robustness than non-cross linked materials.Although they can be readily processed from solution, cross linkingrenders the layer insoluble to further solvent exposure. Cross linkedpolymers may in some cases also have good insulating qualities.

The fluorinated dielectrics disclosed in U.S. Pat. No. 6,265,243 alsopossess many of the desirable characteristics. As such, one possibilitythat the present applicant has considered is to combine the teachings ofthese documents such that the crosslinkable polymers disclosed in U.S.Pat. No. 7,279,777 are provided with a fluorinated silane surfacetreatment. However, the present applicant has realized that even withthis modification, all the desired requirements are not fully met.Instead, the present applicant has realized that it would be better tocombine the advantageous properties of crosslinked polymers, such asthose disclosed in U.S. Pat. No. 7,279,777, with the advantageousproperties of fluorinated polymers.

Fluorinated polymers are more readily solution processable than thefluorinated silanes disclosed in U.S. Pat. No. 6,265,243 and can bereadily deposited from solution by, for example, spin coating or inkjetprinting without damaging underlying layers. Fluorinated polymers can bedeposited from solvents such as fluorinated solvents which areorthogonal to the solvents that dissolve most organic semiconductors.Accordingly, the organic semiconductor is not re-dissolved whendepositing the fluorinated polymer thereover in a top gate structure,leading to a distinct, high quality OSC/dielectric interface.Furthermore, fluorinated polymers provide excellent surface propertiesproviding a better interface with the organic semiconductor. Inparticular, the lack of polar surface groups, and resistance to watervapour that might induce these, leads to improved OTFT properties.

While U.S. Pat. No. 6,265,243 does mention dielectric polymeric materialrich in fluoroalkyl chains, the documents teaches that the dielectricmay be replaced with a dielectric polymeric material rich in fluoroalkylchains as an alternative to treating the dielectric surface withfluorinated silane. As such, this document teaches away from the presentapplicant's inventive concept of combining the features of a fluorinatedpolymer with those of a crosslinked polymer.

In light of the above, in accordance with a first aspect of the presentinvention there is provided an organic thin film transistor comprising:a substrate; a source electrode and a drain electrode disposed over thesubstrate with a channel region therebetween; a layer of organicsemiconductor disposed in the channel region; a gate electrode; and agate dielectric disposed between the layer of organic semiconductor andthe gate electrode, wherein the gate dielectric comprises a cross-linkedpolymer and a fluorine containing polymer.

The cross-linked polymer and the fluorine containing polymers may beprovided in separate layers with the fluorine containing polymerdisposed inbetween the layer of organic semiconductor and thecross-linked polymer layer. Preferably the cross-linked polymer layer isthicker than the fluoropolymer layer. However, the thickness of thelayers will depend on the dielectric constant of the materials used.Accordingly, in certain embodiments the fluoropolymer layer may bethicker than the cross-linked polymer layer. The fluoropolymer layer mayhave a layer thickness in the range 50-300 nm. The cross-linked polymerlayer may have a layer thickness in the range 100-300 nm.

Alternatively, a single layer comprising both cross-linking and fluorinemay be provided. The single layer may have a thickness in the range50-400 nm. In the single layer embodiment, the cross-linking andfluorine may be provided in separate polymers in a blend or they may beprovided in the same polymer, i.e. a cross-linked fluoropolymer. If aseparate cross-linked polymer and a separate fluoropolymer are provided,the concentration of the cross-linked polymer and the fluoropolymer mayvary through the dielectric with a higher concentration of fluoropolymeradjacent the organic semiconductor. This may be achieved by depositingthe polymer from solution in a blend and allowing the polymers to atleast partially phase separate as the solvent evaporates. Partial phaseseparation will result in a single layer with variable concentrations ofpolymer components. Total phase separation results in two separatelayers being formed. Alternatively, the two polymers may be deposited inseparate steps, one on top of the other.

The fluoropolymer may have a k value in the range 1.9-2.3.

According to one embodiment, the organic thin film transistor is abottom gate transistor in which the gate electrode is disposed over thesubstrate, the gate dielectric is disposed over the gate electrode, andthe source and drain electrodes and the layer of organic semiconductorare disposed over the gate dielectric.

According to an alternative embodiment, the organic thin film transistoris a top gate transistor in which the source and drain electrodes aredisposed over the substrate, the organic semiconductor is disposed overthe source and drain electrodes, the gate dielectric is disposed overthe organic semiconductor, and the gate electrode is disposed over thegate dielectric.

The dielectric may also be required to serve as an insulating spacerbetween rows and columns defined by the gate metallisation and sourcedrain metallisation in, for example, a light-emissive display. In thiscase the dielectric according to embodiments of the present inventioncan reduce parasitic capacitance, leakage and the probability of shortsat row/column crossing points. Furthermore, in some regions latermetallisation such as a cathode electrode layer in a light-emissivedisplay can be well spaced from metallisation on the substrate by thedielectric of embodiments of the present invention.

In accordance with a second aspect of the present invention there isprovided a method of manufacturing a top gate organic thin filmtransistor, the method comprising: providing a substrate comprising asource electrode and a drain electrode with a channel regiontherebetween; depositing a layer of organic semiconductor in the channelregion; depositing a gate dielectric over the layer of organicsemiconductor; and forming a gate electrode over the gate dielectric,wherein the gate dielectric comprises a cross-linked polymer and afluorine containing polymer.

In accordance with a third aspect of the present invention there isprovided a method of manufacturing a bottom gate organic thin filmtransistor, the method comprising: providing a substrate comprising agate electrode; depositing a gate dielectric over the gate electrode;forming a source electrode and a drain electrode over the gatedielectric with a channel region therebetween; and depositing a layer oforganic semiconductor in the channel region, wherein the gate dielectriccomprises a cross-linked polymer and a fluorine containing polymer.

In accordance with fourth aspect of the present invention there isprovided an organic thin film transistor comprising: a substrate; asource electrode and a drain electrode disposed over the substrate witha channel region therebetween; a layer of organic semiconductor disposedin the channel region; a gate electrode; and a gate dielectric disposedbetween the layer of organic semiconductor and the gate electrode,wherein the gate dielectric comprises a cross-linked polymer and apolymer having a dielectric constant k in the range 1.9 to 2.3.

In accordance with a fifth aspect of the present invention there isprovided a method of manufacturing a top gate organic thin filmtransistor, the method comprising: providing a substrate comprising asource electrode and a drain electrode with a channel regiontherebetween; depositing a layer of organic semiconductor in the channelregion; depositing a gate dielectric over the layer of organicsemiconductor; and forming a gate electrode over the gate dielectric,wherein the gate dielectric comprises a cross-linked polymer and apolymer having a dielectric constant k in the range 1.9 to 2.3.

In accordance with a sixth aspect of the present invention there isprovided a method of manufacturing a bottom gate organic thin filmtransistor, the method comprising: providing a substrate comprising agate electrode; depositing a gate dielectric over the gate electrode;forming a source electrode and a drain electrode over the gatedielectric with a channel region therebetween; and depositing a layer oforganic semiconductor in the channel region, wherein the gate dielectriccomprises a cross-linked polymer and a polymer having a dielectricconstant k in the range 1.9 to 2.3.

Embodiments of the second to sixth aspects may have one or more of thefeatures described in relation to embodiments of the first aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in further detail, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 shows a top-gate organic thin film transistor structure accordingto a prior art arrangement;

FIG. 2 shows a bottom-gate organic thin film transistor structureaccording to a prior art arrangement;

FIG. 3 shows a top-gate organic thin film transistor structure accordingto an embodiment of the present invention; and

FIG. 4 shows a bottom-gate organic thin film transistor structureaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The top-gate organic thin film transistor structure shown in FIG. 3 isessentially the same as that shown in FIG. 1 but with the dielectriccomprising a primary dielectric layer 10 a adjacent the organicsemiconductor 8 and a secondary dielectric layer 10 b between theprimary dielectric layer 10 a and the gate electrode 12. The primarydielectric layer 10 a is a fluorine containing polymer and the secondarydielectric layer 10 b is a cross-linked polymer.

Similarly, the bottom-gate organic thin film transistor structure shownin FIG. 4 is essentially the same as that shown in FIG. 2 but with thedielectric comprising a primary dielectric layer 10 a adjacent theorganic semiconductor 8 and a secondary dielectric layer 10 b betweenthe primary dielectric layer 10 a and the gate electrode 12. Again,primary dielectric layer 10 a is a fluorine containing polymer and thesecondary dielectric layer 10 b is a cross-linked polymer.

Embodiments of the present invention provide a bi-layer dielectricsystem for organic TFTs comprising:

-   -   1. A primary fluoropolymer dielectric layer in contact with the        OSC. The OTFT channel is formed at the interface between the OSC        and this low k dielectric layer. The fluoropolymer layer also        provides good resistance to moisture and other chemical        contaminants (such as the by-products of cross linking of the        secondary dielectric layer) and is deposited by orthogonal        fluoro-solvents to the OSC.    -   2. A secondary dielectric layer, between the first dielectric        layer and the gate metal. This layer is a cross linked material        with excellent thin film insulator properties. This layer        reduces gate leakage and improves physical robustness. The        second dielectric layer is also chosen to have a higher surface        energy than the fluoropolymer, to promote adhesion of the gate        metal to the dielectric surface.

Embodiments of the present invention allow the advantages offluoropolymers (stability, orthogonal solvents, low k) to be combinedwith those of cross linked materials (high resistivity, robustness). Inparallel, the invention mitigates against the disadvantages offluoropolymer dielectrics (increased gate leakage, poor adhesion togate) and cross linked materials (interaction of cross linkingby-products with the OSC).

Fluoropolymers show great promise as dielectrics for organic TFTs forseveral reasons:

-   -   1. The low k surface leads to improved stability and reduced        sensitivity to, for example, ambient moisture that can damage        OTFT performance.    -   2. As fluoropolymers dissolve in fluorinated solvents, which are        orthogonal to the solvents that dissolve most OSCs, top gate        OTFTs can be readily fabricated without re-dissolving the OSC,        leading to a high quality OSC/dielectric interface.

3. Specifically for bottom gate, low k dielectrics can also improve themorphology of OSC materials that are deposited on top of them. The lowcontact angle surface prevents pinning of OSC molecules to the surfacemaking them more free to crystallise.

However, problems experienced using fluoropolymer dielectrics include:

-   -   1. Although fluoropolymers are generally known for their high        resistivity, the present applicant has found increased gate        leakages when using certain fluorinated dielectrics. This may be        a result of pin-holes in the thin-cast fluoropolymer dielectric        films, or residual fluoro-solvents remaining from the casting        process.    -   2. The low surface energy of fluoropolymers gives rise to issues        of adhesion of the gate metal to the surface of the gate        dielectric

In contrast, the present applicant has found that cross linkeddielectrics typically give rise to reduced gate leakage. Cross linkeddielectrics are more commonly used in bottom gate devices, in which thedielectric is processed before OSC deposition. This is, in part, due tothe fact that the by-products of the cross linking reaction can damagethe OSC if the dielectric is used in a top gate device, and particularlythe OSC dielectric interface.

The bi-layer dielectric system of embodiments of the present inventionavoids the aforementioned problems while providing all the advantages. Amethod of forming an OTFT with top gate architecture involves thefollowing steps:

-   -   1. OSC is deposited onto a substrate with pre-defined        source/drain contacts. The substrate may include a test cell or        backplane array. Drying and annealing are carried out as        required for the particular OSC.    -   2. The “primary” dielectric layer is deposited on the OSC by        e.g. spin coating or ink jet printing. The primary dielectric is        preferably a fluorinated polymer, such as Teflon or CYTOP,        deposited from a fluorinated solvent. A drying step may be        required to remove the solvent.    -   3. An intermediate adhesion promotion layer may be included        between the secondary and primary dielectrics. An example of        such as adhesion promotion layer is PVB (Polyvinyl butyral).    -   4. The “secondary” dielectric layer is deposited on top of the        “primary” dielectric layer. This layer is a cross linked layer,        such as a BCB (Benzocyclobutene), cross-linked PVP (cross linked        with e.g. dichlorosilanes or poly melamine formaldehyde) or a        poly norbonene based system. The primary, fluoropolymer layer        protects the OSC from damage caused by the by-products of the        cross-linking process. Cross linking may be driven thermally        (lower temperatures are preferred to reduce any damage to the        OSC and the fluoropolymer dielectric), chemically, or by        electro-magnetic radiation, such as UV or IR.    -   5. The gate metal is deposited on top of the secondary        dielectric layer.

In an alternative arrangement, a cross linked fluoropolymer may performboth the functions of the two layers described above. A suitablematerial is Poly(pentafluorostyrene-co-glycidyl methacrylate:

The organic semi-conductive material may be solution processable suchthat it can be deposited from solution by, for example, spin-coating orink-jet printing, either directly or in the form of a precursor. Theorganic semi-conductive material may comprise a polymer, oligomer ordendrimer as these have proved to be good solution processablematerials. Small molecule OSC materials such as a soluble pentacenederivative may also be used. Many such semi-conductive materials areknown in the art, for example, conjugated polymers utilized in organiclight-emissive devices. The OSC may also be deposited by vacuumdeposition, as used to deposit pentacene.

The gate electrode can be printed or deposited using other simplepatterning techniques which are known in the art.

The bilayer dielectric may be utilised to reduce track leakage betweenlower and higher level metallization in a device such as a display (e.g.an active matrix organic light-emissive display).

Further details of materials and processes applicable to the inventionare set out below.

Substrate

The substrate may be rigid or flexible. Rigid substrates may be selectedfrom glass or silicon and flexible substrates may comprise thin glass orplastics such as poly(ethylene terephthalate) (PET),poly(ethylene-naphthalate) PEN, polycarbonate and polyimide.

The organic semiconductive material may be made solution processablethrough the use of a suitable solvent. Exemplary solvents include mono-or poly-alkylbenzenes such as toluene and xylene; tetralin; andchloroform. Preferred solution deposition techniques include spincoating and ink jet printing. Other solution deposition techniquesinclude dip-coating; roll printing; and screen printing. In cases wherea solution is dispensed from a nozzle, the printing process may beeither continuous or discontinuous. For example, in a continuous processa continuous strip of organic semiconductive material may be dispensedfrom a nozzle, whereas discontinuous drops are dispensed from a nozzlein a discontinuous printing process.

Organic Semiconductor Materials

Preferred organic semiconductor materials include small molecules suchas optionally substituted pentacene; optionally substituted polymerssuch as polyarylenes, in particular polyfluorenes and polythiophenes;and oligomers. Blends of materials, including blends of differentmaterial types (e.g. a polymer and small molecule blend) may be used.

Source and Drain Electrodes

For a p-channel OTFT, preferably the source and drain electrodescomprise a high workfunction material, preferably a metal, with aworkfunction of greater than 3.5 eV, for example gold, platinum,palladium, molybdenum, tungsten, or chromium. More preferably, the metalhas a workfunction in the range of from 4.5 to 5.5 eV. Other suitablecompounds, alloys and oxides such as molybdenum trioxide and indium tinoxide may also be used. The source and drain electrodes may be depositedby thermal evaporation and patterned using standard photolithography andlift off techniques as are known in the art. A self assembled monolayer,or other surface treatment, may be applied to the source drain contactsto improve charge injection.

Alternatively, conductive polymers may be deposited as the source anddrain electrodes. An example of such a conductive polymers ispoly(ethylene dioxythiophene) (PEDOT) although other conductive polymersare known in the art. Such conductive polymers may be deposited fromsolution using, for example, spin coating or ink jet printing techniquesand other solution deposition techniques discussed above.

For an n-channel OTFT, preferably the source and drain electrodescomprise a material, for example a metal having a workfunction of lessthan 3.5 eV such as calcium or barium or a thin layer of metal compound,in particular an oxide or fluoride of an alkali or alkali earth metalfor example lithium fluoride, barium fluoride and barium oxide.Alternatively, conductive polymers may be deposited as the source anddrain electrodes.

The source and drain electrodes are preferably formed from the samematerial for ease of manufacture. However, it will be appreciated thatthe source and drain electrodes may be formed of different materials foroptimisation of charge injection and extraction respectively.

The length of the channel defined between the source and drainelectrodes may be up to 500 microns, but preferably the length is lessthan 200 microns, more preferably less than 100 microns, most preferablyless than 20 microns.

Gate Electrode

The gate electrode can be selected from a wide range of conductingmaterials for example a metal (e.g. gold) or metal compound (e.g. indiumtin oxide). Alternatively, conductive polymers may be deposited as thegate electrode. Such conductive polymers may be deposited from solutionusing, for example, spin coating or ink jet printing techniques andother solution deposition techniques discussed above

Thicknesses of the gate electrode, source and drain electrodes may be inthe region of 5-200 nm, although typically 50 nm as measured by AtomicForce Microscopy (AFM), for example.

Further Layers

Other layers may be included in the device architecture. For example, aself assembled monolayer (SAM) may be deposited on the gate, source ordrain electrodes, substrate, dielectric, or organic semiconductormaterial to promote crystallity, reduce contact resistance, repairsurface characteristics and promote adhesion where required. Exemplarymaterials for such a monolayer include chloro- or alkoxy-silanes withlong alkyl chains, eg octadecyltrichlorosilane.

OTFT Applications

OTFTs according to embodiments of the present invention have a widerange of possible applications. One such application is to drive pixelsin an optical device, preferably an organic optical device. OTFTs areparticularly suited for use with active matrix organic light emittingdevices, e.g. for use in display applications. An organic light-emittingdevice comprises an anode, a cathode and a layer comprising an organiclight-emitting material therebetween that emits light when a potentialdifference is applied across the anode and the cathode. In addition tothe layer of light emitting material, charge transporting or blockinglayers, or exciton blocking layers, may be provided between the anodeand cathode.

In addition, OTFTs may be used in an active matrix photodetector array,using organic photoresponsive devices. Such an array may be used as animage scanner or X-ray imaging device if used in conjunction with ascintillator material.

Organic thin film transistors according to embodiments of the presentinvention have many common structural features with organiclight-emissive displays and may be formed using similar techniques andmaterials. For example, the dielectric of the present invention may beformed of the same material as that used for bank structures definingpixels of an organic light-emissive display and may thus be formed as acommon layer. As such, according to one advantageous arrangement, theorganic thin film transistors and the organic light-emissive pixels ofan active matrix organic light-emissive display are formed on a commonsubstrate and the dielectric discussed herein forms the bank structurefor the organic light-emissive pixels.

It will be appreciated that pixel circuits comprising an OTFT and anoptically active pixel area (e.g. light emitting or light sensing pixelarea) may comprise further elements. In particular, an OLED pixelcircuit may comprise an OTFT according to the invention as a drivingtransistor and will typically comprise at least one further (organic orinorganic) transistor in addition to the driving transistor, and atleast one capacitor.

While this invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and detail may bemade therein without departing from the scope of the invention asdefined by the appended claims.

1. An organic thin film transistor comprising: a substrate; a sourceelectrode and a drain electrode disposed over the substrate with achannel region therebetween; a layer of organic semiconductor disposedin the channel region; a gate electrode; and a gate dielectric disposedbetween the layer of organic semiconductor and the gate electrode,wherein the gate dielectric comprises a cross-linked polymer and afluorine containing polymer, wherein the gate dielectric comprises asingle layer comprising both the cross-linked polymer and the fluorinecontaining polymer, and wherein the cross-linked polymer and thefluorine containing polymer are provided as separate polymers.
 2. Anorganic thin film transistor according to claim 1, wherein thecross-linked polymer and the fluorine containing polymer vary inconcentration through the gate dielectric with a higher concentration offluorine containing polymer adjacent the organic semiconductor, theconcentration decreasing with increasing distance from the layer oforganic semiconductor.
 3. An organic thin film transistor according toclaim 1, wherein the organic thin film transistor is a bottom gatetransistor in which the gate electrode is disposed over the substrate,the gate dielectric is disposed over the gate electrode, and the sourceand drain electrodes and the layer of organic semiconductor are disposedover the gate dielectric.
 4. An organic thin film transistor accordingto claim 1, wherein the organic thin film transistor is a top gatetransistor in which the source and drain electrodes are disposed overthe substrate, the organic semiconductor is disposed over the source anddrain electrodes, the gate dielectric is disposed over the organicsemiconductor, and the gate electrode is disposed over the gatedielectric.
 5. A method of manufacturing a top gate organic thin filmtransistor, the method comprising: providing a substrate comprising asource electrode and a drain electrode with a channel regiontherebetween; depositing a layer of organic semiconductor in the channelregion; depositing a gate dielectric over the layer of organicsemiconductor; and forming a gate electrode over the gate dielectric,wherein the gate dielectric comprises a cross-linked polymer and afluorine containing polymer, wherein the gate dielectric comprises asingle layer comprising both the cross-linked polymer and the fluorinecontaining polymer, and wherein the cross-linked polymer and thefluorine containing polymer are provided as separate polymers.
 6. Amethod of manufacturing a bottom gate organic thin film transistor, themethod comprising: providing a substrate comprising a gate electrode;depositing a gate dielectric over the gate electrode; forming a sourceelectrode and a drain electrode over the gate dielectric with a channelregion therebetween; and depositing a layer of organic semiconductor inthe channel region, wherein the gate dielectric comprises a cross-linkedpolymer and a fluorine containing polymer, wherein the gate dielectriccomprises a single layer comprising both the cross-linked polymer andthe fluorine containing polymer, and wherein the cross-linked polymerand the fluorine containing polymer are provided as separate polymers.7. A method according to claim 5, wherein a cross-linkable polymer andthe fluorine containing polymer are deposited from solution in a blend.8. A method according to claim 7, wherein the cross-linkable polymer andthe fluorine containing polymer partially phase separate afterdeposition.
 9. A method according to claim 7, wherein the cross-linkablepolymer is cross-linked after being deposited to form the cross-linkedpolymer.
 10. A method according to claim 7, wherein the fluorinecontaining polymer and the organic semiconductor are deposited fromorthogonal solvents.
 11. An organic thin film transistor comprising: asubstrate; a source electrode and a drain electrode disposed over thesubstrate with a channel region therebetween; a layer of organicsemiconductor disposed in the channel region; a gate electrode; and agate dielectric disposed between the layer of organic semiconductor andthe gate electrode, wherein the gate dielectric comprises a cross-linkedpolymer and a polymer having a dielectric constant k in the range 1.9 to2.3, wherein the gate dielectric comprises a single layer comprisingboth the cross-linked polymer and the polymer having a dielectricconstant k in the range 1.9 to 2.3, and wherein the cross-linked polymerand the polymer having a dielectric constant k in the range 1.9 to 2.3are provided as separate polymers.
 12. A method of manufacturing a topgate organic thin film transistor, the method comprising: providing asubstrate comprising a source electrode and a drain electrode with achannel region therebetween; depositing a layer of organic semiconductorin the channel region; depositing a gate dielectric over the layer oforganic semiconductor; and forming a gate electrode over the gatedielectric, wherein the gate dielectric comprises a cross-linked polymerand a polymer having a dielectric constant k in the range 1.9 to 2.3,wherein the gate dielectric comprises a single layer comprising both thecross-linked polymer and the polymer having a dielectric constant k inthe range 1.9 to 2.3, and wherein the cross-linked polymer and thepolymer having a dielectric constant k in the range 1.9 to 2.3 areprovided as separate polymers.
 13. A method of manufacturing a bottomgate organic thin film transistor, the method comprising: providing asubstrate comprising a gate electrode; depositing a gate dielectric overthe gate electrode; forming a source electrode and a drain electrodeover the gate dielectric with a channel region therebetween; anddepositing a layer of organic semiconductor in the channel region,wherein the gate dielectric comprises a cross-linked polymer and apolymer having a dielectric constant k in the range 1.9 to 2.3, whereinthe gate dielectric comprises a single layer comprising both thecross-linked polymer and the polymer having a dielectric constant k inthe range 1.9 to 2.3, and wherein the cross-linked polymer and thepolymer having a dielectric constant k in the range 1.9 to 2.3 areprovided as separate polymers.
 14. An organic thin film transistoraccording to claim 2, wherein the organic thin film transistor is abottom gate transistor in which the gate electrode is disposed over thesubstrate, the gate dielectric is disposed over the gate electrode, andthe source and drain electrodes and the layer of organic semiconductorare disposed over the gate dielectric.
 15. An organic thin filmtransistor according to claim 2, wherein the organic thin filmtransistor is a top gate transistor in which the source and drainelectrodes are disposed over the substrate, the organic semiconductor isdisposed over the source and drain electrodes, the gate dielectric isdisposed over the organic semiconductor, and the gate electrode isdisposed over the gate dielectric.
 16. A method according to claim 6,wherein a cross-linkable polymer and the fluorine containing polymer aredeposited from solution in a blend.
 17. A method according to claim 16,wherein the cross-linkable polymer and the fluorine containing polymerpartially phase separate after deposition.
 18. A method according toclaim 17, wherein the cross-linkable polymer is cross-linked after beingdeposited to form the cross-linked polymer.
 19. A method according toclaim 8, wherein the cross-linkable polymer is cross-linked after beingdeposited to form the cross-linked polymer.
 20. A method according toclaim 8, wherein the fluorine containing polymer and the organicsemiconductor are deposited from orthogonal solvents.
 21. A methodaccording to claim 9, wherein the fluorine containing polymer and theorganic semiconductor are deposited from orthogonal solvents.